Paper is made by forming a mat of fibers, normally wood fibers, on a moving wire screen. The fibers are in a dilution with water constituting more than ninety-nine percent of the mix. As the paper web leaves the forming screen, it may be still over eighty percent water. The paper web travels from the forming or wet end of the papermaking machine and enters a pressing section where, with the web supported on a dryer fabric, the moisture content of the paper is reduced by pressing the web to a fiber content of between forty-two and forty-five percent. After the pressing section, the paper web is dried on a large number of steam heated dryer rolls, so the moisture content of the paper is reduced to about five percent.
The dryer section makes up a considerable part of the length of a papermaking machine. The web as it travels from the forming end to the take-up roll may extend a quarter of a mile in length. A major fraction of this length is taken up in the dryer section. As the paper industry has moved to higher web speeds, upwards of four- to five-thousand feet per minute, the dryer section has had to become proportionately longer because less drying is accomplished at each dryer as the paper moves more quickly through the dryers.
One type of dryer, known as a two-tier dryer, has two rows of steam heated dryer rolls four to seven feet in diameter. The dryer rolls in the upper and lower rows are staggered. The paper web runs in a meandering fashion from an upper dryer roll to a lower dryer roll and then on to an upper roll over as many rolls as is required. An upper dryer fabric backs the web as it travels over the upper dryer rolls, and leaves the paper web as it travels to the lower rolls. The upper dryer fabric is turned by dryer fabric reversing rolls spaced between the upper rolls. On the lower dryer rolls the web is supported by a lower dryer fabric, which is also turned between lower dryer rolls by lower dryer fabric reversing rolls. This apparatus advantageously dries first one side and then the other of the web, however, the paper web is unsupported for a length as it passes from the upper dryer rolls to the lower dryer rolls, and from the lower rolls to the upper rolls. Unsupported paper webs present a problem as web speed increases. At higher web speeds, the paper interacts with the air and can begin to flutter. This fluttering can wrinkle and crease the paper web, seriously damaging the quality of the paper produced. Further, the fluttering can lead to tears and web failure, with all the cost and downtime associated with paper lost during the rethreading operation.
A first approach to overcoming this problem was to use a single dryer fabric or a wire which traveled with the paper web over both the upper and lower dryers so that the paper was supported through the open draws. This approach limited paper flutter in the open draws, but, because the blanket was disposed between the paper web to be dried and the lower dryer rolls, the effectiveness of the lower dryer rolls was substantially diminished.
A further dryer development is the single tier of dryer rolls with vacuum reversing rolls disposed therebetween. The vacuum rolls, such as those shown in U.S. Pat. No. 4,882,854 (Wedel, et al.), use vacuum to clamp the edges of the paper to the reversing roll to prevent edge flutter, and use central grooves to allow passage of the trapped boundary layer between the blanket and the reversing rolls.
Single tier dryer systems are successful in increasing the drying is rate and shortening the dryer section of a papermaking machine. It is necessary in order to dry both sides of the web effectively to employ both top felted and bottom felted single tiers of dryers. Bottom felted dryers have the disadvantage in that removing broke from between the dryer fabric and the dryer can be a difficult and time consuming operation. On the other hand, in the top felted dryers, when the dryer fabrics are loosened, broke drops with relative ease out from between the dryer fabric and the dryer rolls. A further possible problem with single tier dryers is the sequential drying of first one side and then the other. When both sides of the sheet are not dried simultaneously curl can develop in the paper due to the effect of drying on the dimensions of the fibers on one side of the sheet as opposed to the still wet fibers on the other which can produce a tendency for the paper web to curl both in the cross machine and in the machine direction.
Problems with curling of the paper web as it moves through the dryer section also arise from the known tendency of a paper web to dry more rapidly at the edges as opposed to the center of the web. This problem of varying moisture profile in a drying paper web has in the past been dealt with by providing additional drying to the center of the web by for example using infrared radiators divided into zones. Alternatively, the edges of the paper have been sprayed with water to increase the moisture content of the edges to that of the center of the web. Another approach is the placement of a steam box which extends across the width of paper web and is connected to a source of steam for applying steam to the web to equalize the properties of the web.
When a papermaking machine dries a web unevenly in the cross machine direction the paper shrinks more at the dryer edges, producing an uneven tension profile which can contribute to a tendency for the web to curl when formed into sheets. Curling of paper is a highly undesirable property particularly in fine paper which is used with new printing and copying methods. Laser printers and photo copiers heat the paper rapidly from a single side. If this heating produces curl in the paper a paper jam may result when the paper is attempted to be fed by the printer or photo copier machine.
What is needed is a shorter dryer section which dries both sides of the web simultaneously and actively controls paper moisture profile in the cross machine direction.